1. Field of the Invention
This invention relates to activated carbon used to treat aqueous systems. More specifically, this invention relates to activated carbon that has been modified to control pH and alkalinity excursions during the start-up phase of activated carbon aqueous adsorption systems.
2. Description of the Prior Art
Activated carbon has been a technically and economically successful material for treating aqueous systems. However, when starting up activated carbon aqueous treatment systems, it is typical to experience unacceptable increases in the pH and/or alkalinity of the effluent. These unacceptable levels of pH or alkalinity can last from several hours to several days. When these excursions occur, the treated water does not meet the standards for distribution to the customer or discharge to the environment. This problem can lead to a significant loss in production, environmental problems, or expensive remedial actions.
The pH/alkalinity excursion phenomenon has been found to occur for various types of water treatment applications such as municipal water, industrial process water, ground water, and home water filter applications. It has been found to exist using various types of activated carbons such as those produced from bituminous coal, subbituminous coal, wood, coconut or peat feedstocks.
The presence of these pH/alkalinity excursions has been a recurring problem throughout the industry for many years. Notwithstanding the productivity losses associated with these excursions, little has been done to overcome or alleviate the problem. pH/alkalinity excursions have been largely tolerated because no solution was known to exist outside of processes detailed in a select group of patents. Because of the growing concern for the environmental problems associated with these excursions as well as the economic losses, it would be desirable to provide another method for preventing or eliminating them.
An example of activated carbon used to treat aqueous systems is disclosed in U.S. Pat. No. 5,021,164 to Gay. Gay discloses oxidized activated carbon, which is used to remove aromatic compounds from aqueous solutions. The oxidized activated carbon is produced by contacting activated carbon with an acidic oxidizing agent such as permanganate, nitric acid and hydrogen peroxide.
Activated carbon, when first put “on-line” in water service, acts as an ion exchange medium on its surface and causes the pH of the outlet water to rise for up to several hundred or thousands of bed volumes. The problem is currently being addressed by methods such as those disclosed in U.S. Pat. No. 5,714,433 to Farmer et al. Farmer discloses a wet activated carbon that has been treated with carbon dioxide, which is optionally followed by treatment by air. The treated wet activated carbon modifies the initial contact pH of the aqueous system such that it is less than about 9.
U.S. Pat. Nos. 5,368,738 and 5,466,378 to Dussert et al. disclose an oxidized activated carbon having a contact pH between 7.1 and 8.2 for use in aqueous treatment systems. The oxidized activated carbon minimizes pH and alkalinity excursions during start-up of the system.